A secondary battery is a battery that can be repetitively recharged. However, if a secondary battery is charged beyond the full charge level, such overcharging can cause deterioration of the overcharged secondary battery. For example, the life span of the secondary battery is typically decreased by overcharging. In addition, a nickel-hydrogen secondary battery is susceptible to weakening as a result of excessive charging. Therefore, it is important to properly detect a full charge level in order to prevent the secondary battery from being overcharged.
The charging characteristics of a secondary battery often depend upon the type of the secondary battery. For example, a nickel-cadmium secondary battery or a nickel-hydrogen secondary battery has a charging characteristic in which the terminal voltage of the secondary battery typically peaks when the secondary battery is fully charged. Such charging characteristics may be monitored to detect a full charge level.
For example, Japanese Utility Model No. SHO 60-13253 describes a charging method in which a charging apparatus monitors the terminal voltage of the secondary battery and indicates a full charge level when a terminal voltage peak is detected. Such systems are hereinafter referred to as peak control systems.
Japanese Patent Laid-open No. SHO 61-288740 describes a charging method which determines that the full charge level has been reached following detection of a decrease of a predetermined voltage .DELTA.V after a peak of the terminal voltage. Such systems are hereinafter referred to as a -.DELTA.V control system.
FIG. 1 depicts the charging characteristics of a secondary battery being charged in accordance with a typical -.DELTA.V control system. The battery which has been previously been discharged to a full discharge level is first charged with a constant current. Soon after charging is initiated, the terminal voltage rapidly rises to a high level. Then, the terminal voltage more gradually increases. Finally, charging is terminated when the terminal voltage of the secondary battery decreases by a predetermined voltage .DELTA.V following a peak (P).
Referring now to FIG. 2, if a secondary battery has been once over discharged, its charging characteristic sometimes shows a small peak (Ps) followed by a decrease of a predetermined voltage .DELTA.V after the small peak (Ps) before the full charge level (Pr) is actually reached as shown in FIG. 2. Therefore, if a peak control system or a -.DELTA.V control system is used to detect the full charge level of a battery having such a charging characteristic, the charging apparatus may mistake the small peak and the decrease of the .DELTA.V after the small peak for the full charge level. As a result, although the charge level has not actually reached the full charge level, charging is mistakenly terminated.
In order to avoid a false detection of the full charge level, the following method is known. During a predetermined time interval following the initiation of charging, the charging apparatus continues charging without monitoring the terminal voltage for a peak followed by a decrease of .DELTA.V. After the predetermined time period has elapsed, the charging apparatus starts monitoring the terminal voltage for the peak followed by the decrease of .DELTA.V. When these events are detected, charging is terminated.
However, this method has the following disadvantage as illustrated in FIG. 3. If the secondary battery to be charged is at or near a full charge level when charging is initiated, the secondary battery will be overcharged. As shown in FIG. 3, even though the terminal voltage exhibits a peak followed by a decrease of .DELTA.V, the predetermined time length will not yet have elapsed and charging will be continued.